As recognized in the present assignee's co-pending U.S. patent application Ser. No. 10/963,190, hard disk drives (HDD) typically include both disk memory and solid state memory referred to as “cache” for temporarily holding data being transferred between the disks and a host computer. Conventionally, the cache is Dynamic Random Access Memory (DRAM), a volatile form of memory that can undergo a significant number of write/erase cycles and that has a very high data transfer rate to and from the disks.
With the advent of mobile computers that are battery powered, some HDDs have been provided wherein the disks spin down (stop rotating) when inactive for a period of time. The main purpose of this is to extend the battery life of the computer. When a disk drive is spun down to a standby mode with just the electronics active, battery power is conserved. As recognized herein, however, when data must be written to the disks, the disks must spin up to enable the write to take place, which consumes a significant amount of battery power. If this occurs frequently, no power saving can be attained.
Accordingly, the present invention understands that one solution is to cache the write data to DRAM, and then destage the cached data to disk at some later time. As further understood herein, because DRAM is volatile memory, the data in the DRAM unfortunately can be lost if the HDD loses power before cached data is destaged to the disks.
Accordingly, it has been proposed to supplement the DRAM memory with non-volatile flash memory in mobile disk drives. Because flash memory is non-volatile, data that is stored in the flash memory will not be lost if power is lost.
As critically recognized herein, however, the data transfer rate of a flash memory is much slower than that of the disk. Specifically, typical data transfer rates of less expensive flash memory can be as slow as 2 MB/s, while data transfer rates directly to the disk (even a comparatively slower one such as might be used in a laptop computer) may exceed 50-100 MB/s. As a consequence, disk drive performance is greatly reduced when using flash memory, particularly when data is written to the HDD in bursts. A second drawback is that flash memory has a limited number of write cycles. With these critical recognitions in mind, the invention herein is provided.